Release 1.5.0

Bump version and abi info (questionable if the latter is needed,
as the -release flag is used, but I see little harm in doing it).

configure.ac

@@ -10,15 +10,15 @@ AC_PREREQ(2.62)
 m4_define([libmypaint_api_major], [1])
 m4_define([libmypaint_api_minor], [5])
 m4_define([libmypaint_api_micro], [0])
-m4_define([libmypaint_api_prerelease], [beta])  # may be blank
+m4_define([libmypaint_api_prerelease], [])  # may be blank
 
 # ABI version. Changes independently of API version.
 # See: https://autotools.io/libtool/version.html
 # https://www.gnu.org/software/libtool/manual/html_node/Updating-version-info.html
 # The rules are fiddly, and are summarized here.
 
 m4_define([libmypaint_abi_revision], [0])  # increment on every release
-m4_define([libmypaint_abi_current], [0])  # inc when add/remove/change interfaces
+m4_define([libmypaint_abi_current], [1])  # inc when add/remove/change interfaces
 m4_define([libmypaint_abi_age], [0])  # inc only if changes backward compat